Fowey EF Report 2020
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Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 WESTCOUNTRY RIVERS TRUST River Fowey ELECTROFISHING SURVEY REPORT - RIVER FOWEY, 2020 0 Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 Report written by Craig Renton Mapping undertaken by Craig Renton Checked by Bruce Stockley Westcountry Rivers Trust Rain-Charm House Kyl Cober Parc Stoke Climsland Callington Cornwall PL17 8PH Tel: +44 (0) 1579 372140 Email: [email protected] Web: www.wrt.org.uk Version Details of Revision Prepared by Checked by Approved by Date of Issue Draft For client comment Craig Renton Bruce Stockley Bruce Stockley 11/01/21 Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 Contents Executive Summary ................................................................................................................................. 1 1. Introduction .................................................................................................................................... 1 1.1 Electrofishing Protocols ................................................................................................................ 1 1.2 Life cycle and bottlenecks ............................................................................................................. 3 1.3 Catchment Based Fisheries Conservation Strategy (Defend/Repair/Attack) ............................... 4 2. Site selection ................................................................................................................................... 4 3. Field Sampling and data analysis methods ..................................................................................... 5 4. Results and Discussion .................................................................................................................... 7 5. Recommendations ........................................................................................................................ 19 6. Acknowledgments ......................................................................................................................... 22 Table of figures Figure 1 Diagrams defining salmonid habitat bottlenecks (Summers et al, 1996) ................................. 4 Figure 2 2020 Fowey Salmon Length Frequency Distribution. ............................................................... 6 Figure 3 2020 Fowey Trout Length Frequency Distribution. .................................................................. 7 Figure 4 Total catch data for River Fowey, 2020 .................................................................................... 8 Figure 5 Salmon and trout classifications River Fowey, 2020 ................................................................. 9 Figure 6 Fowey trout fry classification stacks. ...................................................................................... 16 Figure 7 Fowey salmon fry classification stacks .................................................................................... 16 Figure 8 Multi-year average fry numbers for salmon, trout and combined salmonids. ...................... 17 Table 1 Semi-quantitative abundance categories for salmon fry (Crozier & Kennedy, 1994) ............... 6 Table 2 Fowey Salmon & Trout Fry Classifications 2020. ....................................................................... 7 Table 3 Multi-year salmon fry classifications (N.B hatched areas are Environment Agency surveys). 12 Table 4 Multi-year trout fry classification (N.B. hatched areas are Environment Agency surveys) ..... 13 Table 5 Recommended DRA actions for Fowey sub catchments ......................................................... 21 Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 Executive Summary This is the ninth year in which Westcountry Rivers Trust (WRT) have carried out a catchment wide electrofishing report on the River Fowey. The results of the 2020 survey have shown an improvement on some of those from previous years and overall the catchment has performed well. The Water For Growth project (W4G) continues work within the catchment for improvements to fish passage and riparian and in-river habitat works. With the project entering its final year some of the information from these surveys will be used to guide efforts and work within the catchment. 1. Introduction WRT has undertaken semi-quantitative fry index electrofishing surveys throughout the River Fowey as part of its annual monitoring program. This was the fourth year of catchment wide monitoring by WRT which commenced as part of the monitoring for the W4G but the tenth consecutive year where previous surveys were enabled by donations from Fowey Rivers Association (FRA). Due to the global pandemic that is affecting many organisations WRT have had to adapt to a modified, operator safeguarding survey methodology which was incorporated into this year’s fry index surveys. Surveys were undertaken between June and September 2020 and a total of seven sites were electrofished by WRT on the Fowey. Surveys were identified to allow for catchment wide coverage with monitoring based on sites historically fished by the Environment Agency with older data available for comparison, those where W4G outputs required monitoring and locations requested from local angling interests. 1.1 Electrofishing Protocols Electrofishing uses a controlled electric current to induce fish to swim toward an anode and into a hand net, and thereby be counted and assessed. When carried out correctly by experienced and qualified surveyors it is not harmful to fish and the fish are released back to the same location they were caught. In upland streams and shallower sections of rivers, an electrofishing backpack is used and therefore this type of kit was used for all the Fowey surveys. There are several approaches to electrofishing assessments in rivers; quantitative, area semi- quantitative and time semi-quantitative methodologies. All three methods have their advantages and disadvantages. Quantitative electrofishing is a thorough methodology that has the highest degree of accuracy of all the methods. The main disadvantages of this approach are it is less mobile than backpack equipment 1 Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 and it takes longer to undertake surveys. It Is therefore more costly than other approaches. With this method, an area of river is netted off and the fish are removed from this defined stretch in multiple passes until sufficient fish are removed to form a very accurate assessment of species and numbers. It is not required to remove all the fish from the area but rather ensure a consistent fishing method that gives a linear decrease in the number of fish caught per pass. The overall catch decline gives an accurate estimate of the total number of fish in the location. This is known as the ‘depletion’ methodology. An area-based semi-quantitative electrofishing methodology follows the same process as quantitative electrofishing but only a single pass is carried out. A lack of multiple passes renders the method only semi quantitative and therefore less accurate, but it has the advantage of being much quicker than the depletion method, and it is suitable for use on all waterbody types. It is able to detect multiple species and is reasonably accurate but is less time efficient and therefore costlier than a time- based methodology (described below). A time-based, semi-quantitative electrofishing methodology differs from both the approaches described above. Instead of limiting the area fished (by use of nets) it limits the amount of time used to fish to assess fish numbers. As no nets are deployed, fish in deeper sections of large rivers can frequently avoid capture using this method. It is therefore only suitable to assess salmonid fry, who are restricted to a shallower section of upland streams and rivers. This method is extremely rapid and therefore cost-effective, allowing for deployment across whole river catchments although its major drawback is its lower accuracy than netted approaches. In weighing up the pros and cons of the various approaches it is worth considering what would be required for a truly reliable method. In scientific publications it is usually considered that if an approach is accurate 95% of the time then this is an acceptable standard. Such an approach would be said to have sufficient statistical power to answer the question asked, for example, ‘has this habitat improvement resulted in more fish in the area studied?’. For an electrofishing methodology to have sufficient statistical power it requires a large number of sites to be fished in a fully-quantitative depletion methodology over a number of years. As such an effort is rarely practicable and will cost more than the habitat improvements it attempts to measure, this approach is rarely applied in the UK. River managers in the UK have limited budgets and therefore it is the case that an electrofishing programme of insufficient statistical power to achieve 95% confidence is usually accepted as a compromise between accuracy and cost. Bearing in mind the limits of statistical power that these approaches usually have (as practically applied), it is important to consider the aim of a given electrofishing programme. In the case of WRT’s 2 Westcountry Rivers Trust Electrofishing Survey Report - River Fowey, 2020 catchment scale electrofishing programme, the aim is to build up historical data on each catchment to provide